This invention relates to polyurethane shoe sole compositions which have the ability to dissipate a static electrical charge.
Elastomeric polyurethanes are widely used in the shoe industry to prepare soles for sports, leisure and dress shoes. For various reasons, it is desirable to provide a shoe which is antistatic. For example, shoes worn by persons who work with or handle electronic equipment are desirably antistatic, as antistatic footwear would reduce the possibility of static discharges occurring between the worker and other objects. These static discharges are often very damaging to electronic components.
In addition, certain medical applications and clean room operations require a dust-free environment. Antistatic footwear would reduce the tendency for dust and dirt to be introduced through electrostatic attraction to the wearer.
Another use is in explosive environments, where it is essential to prevent sparks.
Moreover, antistatic footwear would be desirable in everyday use, particularly in dry climates, to reduce annoying electrostatic discharges between the wearer and surrounding objects.
Several attempts have been made to render polyurethanes antistatic. For example, it is known to use topical antistatic agents such as quaternary ammonium compounds and surfactants to impart surface conductivity to the polyurethane. However, these agents are quickly and easily scuffed off in applications such as shoe soles. It is also known to incorporate conductive fillers and fibers into the polyurethane, but such fillers tend to alter the physical properties and processing characteristics of the polyurethane, rendering them unsuitable for the desired applications. These fillers and fibers must also be used in relatively large quantities, which often makes them relatively expensive.
In U.S. Pat. No.4,618,630 it is taught to render polyurethanes and other polymers antistatic by incorporating therein a certain ionizable salt in conjunction with an "enhancer" compound which augments the conductivity provided by the salt. The enhancer compound is a certain carboxylic acid ester or salt of a fatty acid. This antistatic additive provides excellent static dissipative properties to polyurethanes and other polymers. However, in some instances, the enhancer compound acts as a plasticizer for the polyurethane, altering its properties in an undesirable manner. For this reason, the antistatic agent described in that application is not preferred.
Accordingly, it is desirable to provide an antistatic polyurethane shoe sole which has excellent static dissipative properties. In such shoe sole, it is desirable to employ an antistatic agent which can be used in relatively low levels, and which does not significantly adversely affect the physical properties of the polyurethane.